The purpose of this test is to evaluate the correct operation of an inductive style ABS sensor based upon output voltage and frequency in relation to the speed of the road wheel.
Connection for diagnostic work will vary dependent on application.
Technicians should whenever possible gain access to the test circuit without damage to seals and insulation. If this is not possible then make sure appropriate repairs are completed.
General connection advice
PicoScope offers a range of options within the test kits.
Dependent on difficulty of access, choose from:
Testing sensors and actuators (to include relevant circuit/connectors):
Access to individual wheel-speed sensors may be difficult.
All wheel-speed sensors connect to the ABS control module, which is usually located in the engine bay.
Manufacturer data is required to identify the sensor pins at the ABS control module multi-plug.
NOTE The PicoScope can be connected to gather waveforms from 2 or more wheel-speed sensors during a road test. In these conditions, the computer and screen must be out of the driver's view and/or operated by a second technician.
The rate and size of waveform oscillation shown in Channel A in Example 1 are approximately uniform (there are no gaps or atypical peaks or troughs) with an approximately constant wheel rotation speed.
Conversely, the oscillations in the additional Channel B waveform in Example 2 show a periodically (about every half a second) occurring non-uniformity, where, for a couple of oscillations, the amplitude is greater than the waveform average. The periodic nature of the non-uniformity suggests an issue with the rotating component of the sensor apparatus (i.e. the reluctor ring).
Example 3 shows waveforms simultaneously gathered for all four wheel-speed sensors. Channels A and B remain connected to the same sensors as in the examples above, hence Channel A, Channel C and Channel D are examples of good waveforms, whereas Channel B shows the waveform indicating a reluctor ring fault.
ABS and its derivatives (e.g. stability and traction control) are safety-related systems fitted to vehicles, with ATE, Bosch and Bendix being generally recognised manufacturers. All these systems have similar operational strategies, which rely on sensors to encode in their outputs the speeds of each one of the vehicle's four wheels.
Inductive sensors encode this output as an oscillatory waveform, with each cycle having a fixed relationship to the rotational position of the wheel (e.g. in the examples above, the waveform cycles twenty-nine time per wheel revolution). Thus, the quicker the wheel rotates, the quicker the rate at which the waveform cycles. Furthermore, the sensor output is proportional to the wheel rotation speed and, as the wheel speed increases, the waveform amplitude increases.
Therefore, the ABS control module expects similar (within a given tolerance) oscillatory waveforms from all four of the vehicle's wheel-speed sensors. If the brakes are applied and a wheel locks, then the oscillation rate and amplitude of the waveform from that wheel will fall out of tolerance. In the first instance, this triggers a release of pressure to that wheel's hydraulic brake system, then the module will modulate the pressure to bring the rate of wheel rotation in line with the other wheels and until it no longer requires braking assistance.
In a traction control system, wheel-speed sensors are used to check if the oscillation frequency increases, as might occur if a wheel loses grip and spins at a faster rate than the others. In this scenario, the engine power output is reduced until the frequencies from the ABS sensors are equalised, indicating that traction has been regained. Some systems momentarily apply the brake to the spinning wheel to aid traction on the other wheel through differential action.
If the wheel speed signals continuously fall outside of normal parameters then the ABS function may be turned off. A driver warning light will be illuminated and as with any electrical fault on ABS, normal hydraulic braking is maintained.
C0035 - Left Front Wheel Speed Circuit Malfunction
C0040 - Right Front Wheel Speed Circuit Malfunction
C0041 - Right Front Wheel Speed Sensor Circuit Range/Performance (EBCM)
C0045 - Left Rear Wheel Speed Circuit Malfunction
C0046 - Left Rear Wheel Speed Sensor Circuit Range/Performance (EBCM)
C0050 - Right Rear Wheel Speed Circuit Malfunction
C0051 - LF Wheel Speed Sensor Circuit Range/Performance (EBCM)
C0221 - Right Front Wheel Speed Sensor Circuit Open
C0222 - Right Front Wheel Speed Signal Missing
C0223 - Right Front Wheel Speed Signal Erratic
C0225 - Left Front Wheel Speed Sensor Circuit Open
C0226 - Left Front Wheel Speed Signal Missing
C0227 - Left Front Wheel Speed Signal Erratic
C0229 - Drop Out of Front Wheel Speed Signals
C0235 - Rear Wheel Speed Signal Circuit Open
C0236 - Rear Wheel Speed Signal Circuit Missing
C0237 - Rear Wheel Speed Signal Erratic
C0238 - Wheel Speed Mismatch
C0245 - Wheel Speed Sensor Frequency Error
This help topic is subject to changes without notification. The information within is carefully checked and considered to be correct. This information is an example of our investigations and findings and is not a definitive procedure. Pico Technology accepts no responsibility for inaccuracies. Each vehicle may be different and require unique test settings.
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